1. Field of the Invention
This invention relates to a magneto-optic memory utilizing composite magnetic materials exhibiting magneto-optic rotation and, more particularly, to particles such as europium sulfide dispersed throughout a ferromagnetic matrix which are coupled anti-ferromagnetically at their interface.
2. Background of the Prior Art
Magnetic materials for magneto-optic media must have a number of key properties. Magnetic materials must have a Curie point above room temperature, a square perpendicular hysteresis loop at room temperature, a large magneto-optic rotation at the wavelength of interest such as 800 nm for current products and 400 nm for future products and their deposition should be at a low enough temperature to be compatible with both glass and polymer substrates. Magnetic materials must not exhibit grain noise which means they must be either amorphous, microcrystalline or highly oriented crystallographically. A random polycrystalline film will give an unacceptably high grain noise.
One example of a prior art magneto-optical data storage system is described in U.S. Pat. No. 3,475,738 which issued on Oct. 28, 1969 to H. P. Louis et al. wherein a high permanent ferromagnetic metal is covered with a layer of transparent, ferromagnetic, semiconducting material such as a rare-earth chalcogenide under conditions such that these two layers are in a magnetically exchange-coupled relationship. A beam of light is directed through the transparent semiconducting layer wherein it is reflected back through the layer by the ferromagnetic metal and exhibits rotation of the polarized light beam. While ferromagnetic semiconductors such as EuO, EuS or EuSe provide on the order of 800.degree. rotation per decibel of optical energy loss, the Curie points for these materials are below room temperature. For example, the Curie point for EuO is 72.degree. K., the Curie point for EuS is 19.degree. K. and the Curie point for EuSe is 7.degree. K. Thus, if one of these materials was used for the magneto-optic media, the material must be maintained at temperatures which are in the cryogenic temperature range to keep them below their respective Curie temperature.